The present invention generally relates to drum servo circuits for a magnetic recording and/or reproducing apparatus, and more particularly to a drum servo circuit which can perform an accurate drum servo operation in a magnetic recording and/or apparatus employing a 4-head double-gap system, for example.
During recording and normal reproduction on the so-called 4-head double-gap system magnetic recording and/or reproducing apparatus, standard play heads SP(1) and SP(2) are used in a standard play mode, and expanded play heads EP(1) and EP(2) are used in an expanded play mode. For example, in the case where a magnetic tape is originally intended to provide two hours of play in the standard play mode, it is possible to obtain six hours of play with this magnetic tape in the expanded play mode. The heads are mounted on a rotary drum at such positions that the head EP(2) leads the head SP(1) by a predetermined angle (.theta.) in a rotational direction of the rotary drum and the head EP(1) leads the head SP(2) by the predetermined angle (.theta.) in the rotational direction of the drum.
In such a magnetic recording and/or reproducing apparatus, a rotational phase detection pulse signal is obtained by detecting the rotational phase of the rotary drum. However, there is a phase error t1(.theta.) dependent on the predetermined angle (.theta.) between the rotational phase detection pulse signal obtained in the standard play mode and the rotational phase detection pulse signal obtained in the expanded play mode. Hence, in order to control a drum motor which drives the rotary drum based on sampling pulse signals having identical timings in both the standard and expanded play modes, it is necessary to produce a trapezoidal wave signal for each play mode by changing the time constant of a monostable multivibrator depending on the play mode.
In a conventional drum servo circuit, the time constant of the monostable multivibrator is changed by use of a switch which switches over and connects the monostable multivibrator to an appropriate one of variable resistors depending on the play mode. As a result, the variable resistors which are provided externally to the monostable multivibrator must be used, and it is difficult to produce the drum servo circuit in the form of an integrated circuit because of the external elements. In addition, there is a problem in that the delay introduced in the monostable multivibrator is large in the expanded play mode and causes instability of the drum servo operation.
On the other hand, it is possible to conceive a drum servo circuit in which the time constant of a shaping circuit is changed between the standard and expanded play modes, without the use of the variable resistors connected externally to the monostable multivibrator. According to this conceivable drum servo circuit, it is unnecessary to provide the variable resistors, and furthermore, the delay introduced in the expanded play mode is the same as the delay introduced in the standard play mode and is small. For this reason, it is possible to achieve a stable drum servo operation. However, the delay introduced in the shaping circuit during the expanded play mode is determined by a timing of a rotational speed detection pulse signal which is obtained by detecting the rotational speed of the rotary drum. Therefore, there is a problem in that it is impossible to arbitrarily set the delay time in the shaping circuit.